This invention relates to a system for the detection of moving objects within a survey area by microwave diffraction. Such a detection system includes a transmitter and a receiver. The transmitter includes a microwave oscillator, a modulator and a transmitter antenna. The receiver includes a receiving antenna, a detector, an amplifier a filter for the detected signal, an alarm unit connected to the output of the filter, a modulator and a modulation generator for controlling the modulator.
In such a system, the signal emitted by the transmitting antenna is modified by reflection and/or absorption by both stationary and moving objects in the survey area before reaching the receiving antenna, and a change of the signal received by the latter will therefore be an indication that a moving object, such as a person, is present in the survey area.
If such a system is constructed on the basis of conventional components, these must be of very high quality, and therefore expensive, or the system must continually be checked and adjusted to ensure that no mutual drifting of the transmitter and receiver takes place, which may give rise to a false alarm or failing alarm.